Integrated method for producing chlorine

ABSTRACT

The invention relates to a process for preparing chlorine in three process steps, wherein a catalytic oxidation of hydrogen chloride to chlorine over a uranium oxide catalyst is performed in a first process step, the chlorine formed is at least partly removed in a second step, and an electrochemical oxidation of hydrogen chloride to chlorine is performed in a third process step.

The invention relates to a process for preparing chlorine in three process steps, wherein a catalytic oxidation of hydrogen chloride to chlorine over a uranium oxide catalyst is performed in a first process step, the chlorine formed is at least partly removed in a second step, and an electrochemical oxidation of hydrogen chloride to chlorine is performed in a third process step.

A reaction of great industrial interest is the process for catalytic hydrogen chloride oxidation with oxygen, developed by Deacon 1868.

In the past, the Deacon process was forced very much into the background by chloralkali electrolysis. Virtually all chlorine was produced by electrolysis of aqueous sodium chloride solutions.

However, especially with regard to the global growth in demand for chlorine and in view of the lesser growth in demand for sodium hydroxide solution, which constitutes the significant by-product of chloralkali electrolysis, the abovementioned Deacon process is of high economic interest.

This development is to the benefit of the process for preparing chlorine by catalytic oxidation of hydrogen chloride, which is decoupled from the production of sodium hydroxide solution. In addition, hydrogen chloride is obtained as a coproduct in large amounts, for example, in phosgenation reactions, for instance in isocyanate preparation.

The catalytic oxidation of hydrogen chloride to chlorine is an equilibrium reaction. With increasing temperature, the equilibrium position shifts to disfavour the desired chlorine end product.

The catalysts presently used for catalytic oxidation of chlorine in connection with processes related to the abovementioned Deacon process are therefore based on catalyst components which have a high activity for the conversion of hydrogen chloride to chlorine even at low temperatures.

For instance, WO 2007/134726 discloses that catalysts based on ruthenium, palladium, platinum, osmium, iridium, silver, copper or rhenium are suitable for this purpose. WO 2007/134726 also discloses that a product stream which still comprises proportions of hydrogen chloride, water, oxygen and further secondary constituents, for example carbon dioxide, is always also obtained from this first process step according to the prior art. This results, according to WO 2007/134726, in the necessity in the prior art for a further treatment of the product stream, for example by more or less complex adsorption and desorption processes.

WO 2007/134726 therefore discloses that a process comprising a further process step in the manner of an electrochemical oxidation after preceding removal of the hydrogen chloride by condensation from the remaining product stream is advantageous. It is disclosed that a purification of the hydrogen chloride is preferred, since secondary constituents present in the product stream can thus no longer adversely affect the electrochemical oxidation, in that they, for example, no longer coat the electrolysis cell needed for electrochemical oxidation.

WO 2007/134726 does not disclose that a further side effect, which results especially from the use of catalyst components based on ruthenium, does not occur. This is based on the commonly known property of such transition metals as ruthenium to form complexes with secondary constituents of the process gases at elevated temperatures, or themselves to be converted to a volatile form by oxidation. Such complexes are, for instance, those with carbon monoxide, as may also be present in the process gases from the operation of the process disclosed in conjunction with phosgenation processes according to WO 2007/134726. The formation and also the volatility of such compounds is described, for instance, by Goodwin et al. in “Reactive metal volatilization from Ru/Al₂O₃ as a result of Ruthenium Carbonyl formation” (Appl. Catalysis, 1986 24: 199-209). It is also disclosed therein that such volatilization of ruthenium occurs to a noticeable degree even at temperatures from 100° C.

The possibility of the further oxidation of ruthenium to give the volatile compound is described, for instance, by Backmann et al. in “On the transport and speciation of ruthenium in high temperature oxidising conditions” (Radiochim. Acta, 2005 93: 297-304). It is also disclosed therein that, apart from the Ru and RuO₂ phases, all oxides of ruthenium are volatile compounds which are formed in relatively large amounts at temperatures above 800° C. within minutes. At temperatures of up to 500° C. as disclosed in WO 2007/134726, it can therefore be assumed that the formation of the volatile ruthenium species likewise occurs, though not at that rate. In industrial processes in which such processes are conducted, however, operating times of months up to years are entirely customary, and so a noticeable effect can be expected.

The result of this would be that the catalytic oxidation of hydrogen chloride to chlorine would no longer be able to achieve a conversion to a sufficient degree after a short time owing to the loss of catalyst. Moreover, complexes can be deposited on the electrode surfaces by a reduction of the transition metal present therein in the subsequent electrochemical oxidation, which also adversely affects this process step.

According to WO 2007/134726, it is therefore also preferred to perform the catalytic oxidation isothermally. In each case, the catalytic oxidation should be performed within the temperatures of 180° C. to 500° C. Particular preference is given, however, to relatively low temperatures of 220° C. to 350° C.

The process disclosed in WO 2007/134726 is thus disadvantageous because it cannot be conducted at relatively high temperatures without risk of loss of the catalyst from the catalytic oxidation of hydrogen chloride to chlorine.

Since the catalytic oxidation of hydrogen chloride to chlorine, however, is an exothermic reaction, such a temperature increase should always be prevented in a complicated manner in terms of process technology, or leads, in the case of a fault, possibly to the necessity of renewing the catalysts for the catalytic oxidation of hydrogen chloride to chlorine which have been destroyed thereafter.

Moreover, the process of WO 2007/134726 is disadvantageous because it still requires a purification step between the catalytic oxidation and the electrochemical oxidation, in order, for instance, to prevent deposition on the electrolysis cell needed for electrochemical oxidation.

DE 1 078 100 discloses that salts or oxides of the rare earths, of silver and of uranium are also usable as catalysts for the catalytic oxidation of hydrogen chloride to chlorine. It is also disclosed that, at a temperature of 480° C., a catalyst comprising uranium oxide enables a conversion of 62% of the hydrogen chloride to chlorine.

The process disclosed in DE 1 078 100 is disadvantageous since it allows a maximum conversion of 62%. This is caused by factors including the equilibrium position at elevated temperatures.

Proceeding from the prior art, there thus still remains the object of providing a process which allows a conversion of hydrogen chloride to chlorine to be enabled, without being subject to the restrictions according to the prior art processes with regard to necessary purification of the gas stream or with regard to low achievable conversions.

It has now been found that, surprisingly, a process for preparing chlorine from a reaction mixture A comprising at least hydrogen chloride, comprising the steps of

-   -   a) oxidizing hydrogen chloride with oxygen to chlorine in at         least one first reaction zone in the presence of a catalyst to         obtain a product stream P₁,     -   b) at least partly removing the chlorine present in the product         stream P₁ from a) to obtain a product stream P₃,     -   c) electrochemically oxidizing the hydrogen chloride present in         the product stream P₃ from b) in a second reaction zone, the         product stream P₃ also comprising water, and in some cases also         chlorine,         characterized in that the product stream P₃ from b) is fed         directly to the reaction zone in c), and in that the catalyst         in a) comprises a uranium compound, is capable of achieving this         object.

The process according to the invention is particularly advantageous because it has been found that, surprisingly, the process according to the invention allows the direct feeding of the product stream P₃ into the electrochemical oxidation of hydrogen chloride to chlorine without there being any need for further processing apart from the removal of chlorine, such that the reaction mixture fed to step a) of the process may also comprise secondary constituents which need not be removed in step b) of the process.

This is caused by the catalyst comprising a uranium compound, which can be operated at higher temperatures without its catalytic properties being influenced significantly, and because any secondary constituents present in the reaction mixture A can be oxidized at these higher temperatures.

In connection with the present invention, secondary constituents denote substances which comprise carbon and in which the carbon is present at least partly in an oxidation number less than or equal to two.

Examples of secondary constituents in whose presence the process according to the invention is found to be particularly advantageous are therefore, for instance, halogenated or nonhalogenated aromatic hydrocarbons, such as chlorobenzene, o-dichlorobenzene, p-dichlorobenzene, trichlorobenzenes, the corresponding chlorotoluenes or chloroxylenes, chloroethylbenzene, toluene or xylene. Such secondary constituents originate typically from phosgenation processes with which the process according to the invention can thus also be conducted in an integrated system in an advantageous manner.

A further example of a secondary constituent in whose presence the process according to the invention is found to be particularly advantageous is carbon monoxide.

Whereas, in processes, for example, according to the disclosure of WO 2007/134726, it is to be expected that the presence of, for example, carbon monoxide leads to the formation of transition metal complexes in the catalytic oxidation of hydrogen chloride to chlorine and hence, after a short time, to inadequate conversion of the catalytic oxidation, and subsequently likewise to inadequate conversion in the course of the electrochemical oxidation as a result of deposition of elemental transition metal on the electrode of the electrochemical oxidation, the process according to the invention is surprisingly found not to be prone to such problems, since the catalyst comprising a uranium compound does not tend to form such complexes.

It has thus also been found that, surprisingly, the catalyst used in step a) of the process according to the invention, comprising a uranium compound, is in no way discharged, for example in the manner of volatile compounds with secondary constituents, from the reaction zone of step a) of the process according to the invention, such that long-term operation of a process according to the invention can likewise be ensured.

Moreover, it is thus ensured that no metal or transition metal residues can be present in the product stream P₃, and can be deposited on the surface of the electrodes of the electrochemical oxidation in step c) of the process according to the invention.

Catalysts comprising a uranium compound may, in accordance with the present invention, comprise a support material or may not.

When a catalyst comprising a uranium compound and a support material is used in step a) of the process according to the invention, usable support materials are those selected from the list comprising silicon dioxide, aluminium oxide, titanium dioxide, tin dioxide, zirconium dioxide, cerium dioxide, carbon nanotubes or mixtures thereof.

Typically, the proportion of the uranium compound in the catalyst, when it additionally comprises a support material, is in the range from 0.1 to 90% by weight, preferably in the range from 1 to 60% by weight, more preferably in the range from 1 to 50% by weight, based on the total mass of uranium compound and support material.

The use of catalysts comprising a support material is generally advantageous in order, more particularly, to obtain the structured beds described hereinafter. However, the support materials according to the above list, similarly to the catalysts used according to the prior art, may be found in product stream P₁ and/or P₃, and so their use may be disadvantageous compared to the preferred development described below. In general, the support materials have a lesser tendency to be deposited on the electrodes of step c) of the process according to the invention, and so they are nevertheless usable in principle.

Suitable uranium compounds of the catalyst are uranium oxides, uranium chlorides and/or uranium oxychlorides. Suitable uranium oxides are UO₃, UO₂, UO, or uranium oxides of a nonstoichiometric composition. Preferred uranium oxides of nonstoichiometric composition are those with a uranium to oxygen ratio according to the formula UO_(X) of UO_(2.1) to UO₅. Particular preference is given to those uranium oxides selected from the list comprising U₃O₅, U₂O₅, U₃O₇, U₃O₈ and U₄O₉. In connection with the present invention, uranium oxychlorides denote substances of the general composition UO_(x)Cl_(y) where x and y are each natural numbers greater than zero. Uranium oxychlorides thus also denote nonstoichiometric compositions comprising chlorine, oxygen and uranium.

In a preferred development of the process according to the invention, the catalyst used comprises only a support composed of a uranium compound, i.e. the catalyst comprises only a uranium compound.

The use of such catalysts is particularly advantageous because the use of transition metals and noble metals can be dispensed with completely, thus allowing the above disadvantages of the prior art processes with regard to the catalysts used to be ruled out. Moreover, in this case, it is also possible to rule out any disruptive influences of support materials on the electrochemical oxidation in step c). Such disruptive influences are, for instance, the at least partial entrainment of the support materials in the product stream A, which can then in turn be deposited on the surfaces of the electrodes of the electrochemical oxidation in step c) of the process according to the invention.

The process according to the invention can thus be operated over a long period, without any renewal of the catalyst of step a) or of the electrodes of step c) being necessary. This is especially economically advantageous.

The catalyst used may be present as a bed of particles or in the form of shaped bodies.

When the catalyst is present as a bed of particles, it is preferably present as a structured bed, which is characterized in that the catalyst activity rises in the main flow direction of the reaction zone of step a).

This structured bed is particularly advantageous because it allows equal conversions per unit space to be achieved in the main flow direction of the reaction zone of step a). While high reaction rates can already be achieved at the inlet to the reaction zone as a result of the high concentration of hydrogen chloride and oxygen, they are still maintained towards the outlet of the reaction zone as a result of the elevated catalyst activity. This results in particularly efficient use of the catalyst.

Such a structuring of the catalyst bed can be accomplished by different ratios of uranium compound to support material or by different dilution of a catalyst with an inert material.

When the catalyst is present in the form of shaped bodies, suitable shaped bodies have any desired forms, preference being given to tablets, rings, cylinders, stars, wagonwheels or spheres, particular preference to spheres, rings, cylinders or star extrudates.

The reaction zone of step a) of the process according to the invention can be conducted at temperatures above 350° C. up to temperatures of 800° C. It is preferably operated at temperatures of 400 to 600° C.

In contrast to the prior art processes, as described, for instance, in WO 2007/134726, the upper temperature at which the process according to the invention is no longer performable adequately is not a restriction, but merely constitutes a restriction to the effect that the surprising positive effect of the possibility of oxidation of secondary constituents has already occurred almost completely, and so a further increase in the temperature appears to be economically disadvantageous.

The lower temperature limit is particularly advantageous because a multitude of the secondary constituents are oxidized at this temperature in the reaction zone of step a) of the process according to the invention already to further gaseous compounds.

In the case of secondary constituents which comprise only hydrocarbons, such further gaseous compounds are, for instance, carbon monoxide and/or carbon dioxide. In the case of secondary constituents which comprise chlorine, such further gaseous compounds are, for instance, hydrogen chloride and/or chlorine, which are again part of the process according to the invention. The process thus has a particularly advantageous effect when the secondary constituents are halogenated, especially chlorinated, hydrocarbons, or carbon monoxide.

Moreover, the catalyst comprising a uranium compound surprisingly becomes more active at these temperatures than at lower temperatures, which is contrary, for example, to the prior art ruthenium catalysts which tend to be entrained with the product stream P₁ with increasing temperature, and hence lose activity in the course of operation of such processes. This is not the case in the process according to the invention.

Step a) of the process according to the invention is performed typically at pressures between 1 and 30 bar, preferably at temperatures of 5 to 10 bar.

Compared to the above-disclosed preferred temperature ranges, these pressures are not essential for the particularly advantageous performability of the process according to the invention.

Instead, the pressures disclosed here are the ranges within which the general performance of the process according to the invention has been found to be economically viable. However, for example by virtue of the connection of the process according to the invention to further processes in the manner of an integrated process system, lower or higher pressures may also be found to be advantageous, without the process according to the invention losing its particular advantageousness as a result.

Step a) of the process according to the invention can be performed in one or more reaction zones connected in parallel or in series. In this case, the individual reaction zones may be present in one apparatus or else be present divided into different apparatuses.

The oxygen can either be added completely together with the hydrogen chloride upstream of the first reaction zone or distributed over the different reaction zones.

Moreover, step a) of the process according to the invention, independently of step b) of the process according to the invention, can be performed continuously or batchwise. Preferably, step a) of the process according to the invention is, however, performed continuously.

Apparatuses in which step a) of the process according to the invention can be performed are, for instance, fixed bed, moving bed or fluidized bed reactors, the embodiments of which are common knowledge to those skilled in the art. Preference is given to fixed bed reactors, since the aforementioned structured bed of the catalyst can be achieved therein in an advantageous manner.

In a preferred development of step a) of the process according to the invention, the heat generated in the reaction zone by the exothermic formation of chlorine from hydrogen chloride is withdrawn from the product stream P₁ in the reaction zone or downstream of the reaction zone, and used for the heating of the reaction mixture A in or upstream of the reaction zone of step a). This can optionally be done together with the at least partial conversion of the product stream P₁ from step a) to a liquid phase, as described below.

Such a removal of heat is particularly advantageous because it makes the process more economically viable.

The removal in step b) of the process according to the invention is effected typically according to principles relating to the removal of chlorine or hydrogen chloride from gas streams, which are common knowledge to those skilled in the art. Nonexclusive examples thereof are, for instance, fractional or non-fractional condensation of at least hydrogen chloride, or the adsorption and subsequent desorption of chlorine or hydrogen chloride.

It is essential that at least fractions, preferably more than 50% by weight, more preferably more than 80% by weight, of the chlorine present in the product stream P₁ are removed to obtain a product stream P₂, since this tends to form hydrogen chloride in the presence of water in the manner of an equilibrium reaction, which is common knowledge to those skilled in the art. Since the intention is to form chlorine by the process according to the invention, such formation of hydrogen chloride is disadvantageous.

In the context of step c) of the process according to the invention, which should preferably be performed in a liquid phase, it is preferred when the hydrogen chloride present in the product stream P₁ is fractionated in step b), i.e. essentially only the hydrogen chloride and optionally fractions of water are removed from the product stream P₁ by condensation and this condensate forms the product stream P₃, while the remaining chlorine is essentially conducted out of the process in the form of a second product stream P₂.

The electrochemical oxidation of hydrogen chloride to chlorine in step c) of the process according to the invention can be performed by commonly known diaphragm processes or by means of an oxygen-consuming cathode.

Possible embodiments of diaphragm processes are described in WO 2007/134726.

In a preferred embodiment of the process according to the invention, step c) of the process is performed with an oxygen-consuming cathode.

For this purpose, the product stream P₃ from step b) of the process according to the invention is preferably first converted to a liquid phase.

In this preferred embodiment of the process using an oxygen-consuming cathode, a first electrode space E₁ is present in the reaction zone of step c), and the product stream P₃ comprising hydrogen chloride which results from step b) of the process according to the invention is fed thereto, and there is a further electrode space E₂ in the reaction zone, likewise comprising an electrode, into which an electrolyte solution comprising dissolved oxygen or a gas comprising oxygen is introduced, electrode space E₁ and electrode space E₂ being separated by a membrane, and the electrodes being connected to one another in an electrically conductive manner via a power supply S.

The electrode of the electrode space E₁ can be used here in the form of a rod, of a plate, of a mesh or of a fabric, and may consist of a material selected from the list comprising carbon black, graphite or metal. The metals used may, for example, be titanium or titanium alloys, or the specialty metal alloys which are commonly known to those skilled in the art under the names Hastelloy and Incolloy.

Particular preference is given to materials selected from the list comprising graphite, titanium, titanium alloy, or the specialty metal alloys Hastelloy and Incolloy.

The electrodes of the electrode space E₂ may possess the same forms as the electrodes of the electrode space E₁ and consist of titanium or titanium alloys, for example titanium-palladium, and may be coated. When the electrode is coated, it is preferably coated with a mixed oxide comprising one or more of the metals ruthenium, iridium and titanium. Particular preference is given to a coating comprising a mixed oxide composed of ruthenium oxide and titanium oxide, or a mixture of ruthenium oxide, iridium oxide and titanium oxide.

The electrodes of the electrode space E₂ may also consist of graphite and other carbon materials such as diamond.

The aforementioned materials of the electrodes of the electrode spaces E₁ and E₂ are particularly advantageous because they are particularly resistant to the chemically aggressive substances hydrogen chloride and chlorine. This means that these materials do not tend to corrode on contact with the product stream P₃, such that the advantageousness of the process according to the invention is particularly marked, since there is no need to renew catalyst and/or electrodes for a particularly long period and the overall process can thus be operated for a particularly long period without any need for maintenance.

The membrane present between the electrode spaces E₁ and E₂ of the reaction zone of step c) is typically a polymer membrane. Preferred polymer membranes are all polymer membranes which are common knowledge to those skilled in the art by the umbrella term of cation exchange membranes. Preferred membranes comprise polymeric perfluorosulphonic acids. The membranes may also comprise reinforcing fabric of other materials, preferably fluorinated polymers and more preferably polytetrafluoroethylene.

The thickness of the membrane is typically less than 1 mm. The thickness of the membrane is preferably less than 500 μm, more preferably less than 400 μm, most preferably less than 250 μm.

Step c) according to the preferred embodiment is typically conducted with application of a current density of 4-7 kA/m².

Step c) of the process according to the invention can be performed at any desired pressure. However, it is preferably performed at a lower pressure than that of step a) of the process according to the invention. More preferably, step c) is conducted at approximately ambient pressure (1013 hPa).

Step c) of the process according to the invention can likewise be performed at any desired temperatures. However, it is preferably performed at temperatures of room temperature to 100° C.

The temperatures are particularly advantageous because the energy content of the stream obtained from step c) of the process is thus reduced, and so this energy is available to the process according to the invention. This energy was preferably recovered in the form of heat according to the above-described preferred development of step a) of the process, and used for the heating of the reaction mixture A of step a) of the process according to the invention.

The invention is illustrated below with reference to examples and figures, without thus restricting it thereto.

FIG. 1 shows an embodiment of the process according to the invention, in which a reaction mixture (A) comprising hydrogen chloride, oxygen and secondary constituents including carbon monoxide and chlorobenzene or other partly halogenated aromatics such as dichlorobenzene or the like is introduced with a temperature T₂ into a first reaction zone (R₁) comprising a catalyst (K) composed of uranium oxide, after the reaction mixture A has been heated in a first heat transferer (W₁) from a temperature T₁ to T₂>T₁. In the reaction zone R₁, the reaction mixture is heated to a temperature T₃>T₂ and leaves as the first product stream P₁ comprising residues of hydrogen chloride, and also chlorine and carbon dioxide. In a second heat transferer (W₂), the first product stream (P₁) is cooled to a temperature T₄<T₃. The heat obtained here is passed by means of a heat carrier liquid connection (L) to the first heat transferer W₁. In the course of this, the first product stream P₁ is partly condensed in the second heat transferer W₂, such that a second product stream can be conducted out of the process in the form of a gas stream (P₂) comprising essentially chlorine and carbon dioxide. Thereafter, the condensed third product stream (P₃) is fed to a first electrode space (E₁) in a second reaction zone (R₂), the first electrode space E₁ being connected to a second electrode space (E₂) via a membrane (M). In each of the electrode spaces E₁ and E₂, there are graphite electrodes in rod form (1, 2), which are connected to a power source (S). In the second reaction zone R₂, chlorine is formed in the electrode space E₁ by means of electrochemical oxidation from the residue of hydrogen chloride in P₃, while oxygen is simultaneously reduced to water in the second electrode space E₂. A fourth product stream P₄ is obtained from the electrode space E₁ of the second reaction zone R₂, which comprises only chlorine.

EXAMPLES Example 1 Preparation of a Uranium Oxide Catalyst on an Aluminium Oxide Support

In a beaker, 40 g of gamma-Al₂O₃ shaped bodies (BET of 200 m²/g, from Saint Gobain) were impregnated with a 10% aqueous solution of uranyl acetate dihydrate (from Riedel de Haen) by spraying.

After an action time of 1 h, the solid was dried in an air stream at 80° C. for 2 h. The entire experiment was repeated until 12% by weight of uranium is present on the shaped bodies.

Example 2 Preparation of a Catalyst Comprising Only Uranium Oxide

2 g of a pulverulent uranium(V/VI) oxide (from Strem Chemicals) were dried at ambient pressure in a drying cabinet at 150° C. overnight, and then calcined at 500° C. under air for 2 h.

Example 3 (Comparative Example): Preparation of a Ruthenium Catalyst on an Aluminium Oxide Support

5 g of spherical gamma-Al₂O₃ shaped bodies (from Saint-Gobain) with an average diameter of 1.5 mm and a BET surface area of 200 m²/g were impregnated with a solution of 0.258 g of commercial ruthenium chloride n-hydrate (from Riedel de Haen) in 6.5 g of H₂O analogously to Example 1.

After an action time of 1 h, the solid was dried in an air stream at 60° C. for 5 h. Subsequently, the catalyst was calcined at 250° C. for 16 h. This gives a catalyst with, by conversion, 2% by weight of ruthenium.

Example 4 Performance of Step a) of the Process with Catalyst According to Example 1

25 g of the catalyst from Example 1 were introduced into a fixed bed Ni reractor (diameter 22 mm, length 800 mm) together with 25 g of TiO₂ inert material.

This gave a fixed bed of approx. 150 mm. The fixed bed was heated by means of electrical heating so as to form a radial temperature gradient of 400-600° C. At a pressure of 4 bar, a gas mixture of 100 l (STP)/h of hydrogen chloride, 100 l (STP)/h of oxygen and 60 l (STP)/h of nitrogen was passed through the fixed bed reactor.

The product stream was conducted through two condensation vessels, such that the water formed and the remaining hydrogen chloride were condensed out, while the chlorine was removed as a gas stream. After an operating time of 6 h, the condensate formed was analysed for the aluminium and titanium contents by means of ICP-OES (Inductively Coupled Plasma—Optical Emission Spectrometry, instrument: Varian Vista-PRO, method according to manufacturer's instructions) and for the content of uranium by means of ICP-MS (Inductively Coupled Plasma—Mass Spectrometry, instrument: HP Agilent 4500, method according to manufacturer's instructions). From the analysis, the concentrations of aluminium, titanium and uranium in the condensate shown in Table 1 were determined.

TABLE 1 Metal content in the condensate according to Example 4 Metal Concentration [mg/l] aluminium 1.6 titanium 7.9 uranium <0.001

It is evident that the support materials, compared to the prior art catalysts, already have a significantly reduced tendency to form volatile compounds, since the latter are already found in the condensate more highly concentrated at least by a factor of 1000 than the uranium compounds. This shows both the advantageous operation of the process according to the invention compared to the prior art, and especially the advantageousness of the preferred embodiment thereof.

Example 5 Performance of step a) of the Process with Catalyst According to Example 2

0.2 g of the catalyst obtained according to Example 2 was ground and introduced as a mixture with 1 g of quartz sand (100-200 μm) into a quartz reaction tube (diameter of 10 mm).

The quartz reaction tube was heated to 600° C. and operated at this temperature thereafter.

A gas mixture of 80 ml/min of hydrogen chloride and 80 ml/min of oxygen was passed through the quartz reaction tube.

After an operating time of 140 h, the product gas stream formed was passed through a condensation trap for several hours, such that the H₂O formed and unreacted hydrogen chloride were condensed out. The condensate was analysed for the uranium content analogously to Example 3. This gave a uranium concentration of 0.044 mg/l in the condensate.

Example 6 (Comparative Example): Performance of Step a) of the Process with Catalyst According to Example 3

Analogously to Example 5, a quartz reaction tube was charged with 0.2 g of the catalyst from Example 3 and diluted with quartz sand.

The quartz reaction tube was heated to 540° C. and operated at this temperature thereafter.

A gas mixture identical to that of Example 5 was passed through the quartz reaction tube.

After an operating time of 37 h, the product stream formed was passed through a condensation trap for several hours, such that the H₂O formed and unreacted hydrogen chloride were condensed out. The condensate was analysed for the ruthenium and aluminium contents analogously to Example 4.

TABLE 2 Metal content in the condensate according to Example 6 Metal Concentration [mg/l] aluminium 1.5 ruthenium 3.0

It is evident from this that the concentration of the ruthenium in the product stream, even after an operating time of 37 h, at a reduced temperature compared to Example 5, exceeds that of the uranium by a factor of about 68. The amount of aluminium present is comparable to that of Example 4.

This very clearly shows the particular advantageousness of the process according to the invention and more particularly according to the preferred development, since a depletion of the ruthenium in the inventive step c) of the process now need no longer be expected. 

1. A process for preparing chlorine from a reaction mixture A comprising at least hydrogen chloride, comprising the steps of a) oxidizing hydrogen chloride with oxygen to chlorines in at least one first reaction zone in presence of a catalyst to obtain a product stream P₁, b) at least partly removing the chlorine present in the product stream Pi from a) to obtain a product stream P₃, c) electrochemically oxidizing the hydrogen chloride present in the product stream P₃ from b) in a second reaction zone, the product stream P₃ also comprising water, and in some cases also chlorine, wherein the product stream P₃ from b) is fed directly to the reaction zone in c), and in that the catalyst in a) comprises a uranium compound.
 2. The process according to claim 1, wherein the catalyst comprises a support material.
 3. The process according to claim 2, wherein the support material is a uranium compound.
 4. The process according to claim 1, wherein the uranium compounds are uranium oxides, uranium chlorides and/or uranium oxychlorides.
 5. The process according to claim 4, wherein the uranium oxides are UO₃, UO₂, or UO is, or uranium oxides of a nonstoichiometric composition.
 6. The process according to claim 5, wherein the uranium oxides of nonstoichioetric composition are those with a uranium to oxygen ratio according to the formula UO_(x) of UO₂₁ to UO₅.
 7. The process according to claim 1, wherein the step a) is conducted at temperatures above 350° C. up to temperatures of 800° C.
 8. The process according to claim 1, wherein a fractional condensation to obtain the product stream P₃ performed in step b), and from which a product stream P₂ comprising essentially chlorine is removed.
 9. The process according to claim 8, wherein the heat withdrawn from the product stream P₁ is used for the heating of the reaction mixture A in or upstream of reaction zone of the step a).
 10. The process according to claim 1, wherein step c) is performed with an oxygen consumption cathode. 